1. Field of the Invention
The present invention relates to a semiconductor device such as a power switching semiconductor device, for example, and more particularly, it relates to an improvement for maintaining parasitic inductances of main electrode plates serving as paths for main currents at low levels also during operation of the device.
2. Description of the Background Art
FIG. 29 is a front sectional view showing a conventional power switching semiconductor device. As shown in FIG. 29, conductor foils 21a to 12g, which are so patterned as to form wires of a circuit, are bonded onto an upper surface of an insulating plate 21 which is formed by a ceramics plate in this conventional device, for forming a circuit board by the insulating plate 21 and the conductor foils 21a to 2lg. The conductor foils 21a to 21g are substantially made of copper.
Power switching semiconductor elements E1 and E2 are provided on the circuit board. The power switching semiconductor elements E1 and E2, which are transistor elements, for example, are soldered to the conductor foils 21f and 21d respectively in states of unmolded semiconductor chips. Lower ends of main electrode plates 1 and 2 are soldered to the conductor foils 21c and 21b respectively.
The main electrode plates 1 and 2 are plate type electrodes for relaying main currents between the circuit on the circuit board and the exterior of the device. The respective conductor foils and the power switching semiconductor elements E1 and E2, or the conductor foils themselves are properly connected with each other through wires (bonding wires) 25. Referring to FIG. 29, the conductor foil 21c and the power switching semiconductor element E2, the power switching semiconductor element E2 and the conductor foil 21e, the conductor foil 21e and the power switching semiconductor element E1, and the power switching semiconductor element E1 and the conductor foil 21g are connected with each other through the wires 25 respectively. These wires 25 are substantially made of aluminum.
A radiation plate 9 which is made of a heat conductive metal such as aluminum or copper is bonded to the lower surface of the insulating plate 21. This radiation plate 9 is engaged in lower ends of side wall portions of a box-type case 11, to cover an opened lower end portion of the case 11 having the side wall portions and an upper end portion. Consequently, a storage chamber is defined by the case 11 and the radiation plate 9. The bottom surface of the radiation plate 9 is exposed to the exterior, in order to efficiently dissipate heat loss which is caused in circuit parts such as the power switching semiconductor elements E1 and E2.
The circuit parts such as the insulating plate 21, the power switching semiconductor elements E1 and E2 and the wires 25 are stored in the storage chamber, thereby being protected against the exterior. The storage chamber is filled up with two types of resin materials 41 and 42. The resin material 41, which is soft electrical insulating resin such as silicone resin, directly seals the circuit parts such as the power switching semiconductor elements E1 and E2 and the wires 25 provided on the circuit board. Further, the resin material 41 also seals portions of the main electrode plates 1 and 2 which are close to the lower ends.
The other resin material 42, which is hard electrical insulating resin such as epoxy resin, is not in direct contact with the circuit parts on the circuit board other than the main electrode plates 1 and 2, but seals a region of the storage chamber which is not filled up with the resin material 41. Namely, the soft resin material 41 is filled to serve as a buffer material between the hard resin material 42 and the circuit board. These resin materials 41 and 42 are filled up in the storage chamber for sealing the circuit board and the circuit parts provided thereon, thereby preventing penetration of water etc. from the exterior.
The main electrodes 1 and 2 are stored in the storage chamber, while upper ends thereof are outwardly exposed through the upper end portion of the case 11. The upper ends of the main electrode plates 1 and 2 are bent to cover nuts 31, which are embedded in the upper end portion of the case 11. The upper ends of the main electrode plates 1 and 2 are provided in portions opposed to holes of the nuts 31 with through holes, whereby the device can be fastened to an external unit through bolts.
The two main electrode plates 1 and 2 are mounted to be parallel to each other, so that parasitic inductances generated therein are suppressed. The main electrode plates 1 and 2 are further bent in a substantially S-shaped manner, in order to relax stress concentration, caused by thermal deformation of the insulating plate 21 as described later, at the connecting portions between the main electrode plates 1 and 2 and the conductor foils 21c and 21b. In order to effectively attain the effect of relaxing the stress concentration, the substantially S-shaped bent portions are sealed in the soft resin material 41, not to be deformed or interfered by the hard resin material 42.
In this conventional device, the power switching semiconductor elements E1 and E2 are switched on (conduct) and off (cut off) in response to control signals which are inputted therein through control terminals (not shown). In response to the operations of the power switching semiconductor elements E1 and E2, main currents intermittently flow through the main electrode plates 1 and 2. Namely, this device serves as a switching semiconductor device.
In this conventional device, the heat loss which is caused in the power switching semiconductor elements E1 and E2 upon operation of the device is successively conducted from the conductor foils 21f and 21d through the insulating plate 21 and the radiation plate 9, to be dissipated toward an external cooling body (not shown) on which the radiation plate 9 is mounted. At this time, the insulating plate 21 and the radiation plate 9 are thermally distorted due to difference in thermal expansion coefficient therebetween. Consequently, the insulating plate 21 and the radiation plate 9 are deformed to be bent.
The main electrode plates 1 and 2, which are fixed to the insulating plate 21 through the conductor foils 21c and 21b respectively, are also deformed following the thermal deformation of the insulating plate 21. The substantially S-shaped bent portions of the main electrode plates 1 and 2, which are sealed in the soft resin material 41 as hereinabove described, are relatively freely deformed following the thermal deformation of the insulating plate 21. Further, the resin material 41 is also thermally expanded to deform the bent portions which are sealed in the resin material 41. Consequently, the parallel relation between the main electrode plates 1 and 2 cannot be maintained and the effect of reducing parasitic inductances is reduced.
Further, considerable stress concentrically acts on the connecting portions between the main electrode plates 1 and 2 and the conductor foils 21c and 21b, although this stress is relaxed by the substantially S-shaped bent portions. Consequently, the connecting portions may be broken following repetitive employment over a long period.
In the conventional power switching semiconductor device, as hereinabove described, the main electrode plates 1 and 2 are fixed to the insulating plate 21 and hence the parallelism is reduced following employment of the device, to cause parasitic inductances or damage the connecting portions due to stress concentration.